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1、Heat transfer Principles of Chemical Engineering-CDUTOverview of heat transferLearning objectives and requirementsThrough the study of this chapter,you are supposed to master the basic principles and laws of heat transfer,and apply these principles and laws to analyze and calculate the heat transfer
2、 process.Principles of Chemical Engineering-CDUTOverview of heat transferheat transferHeat transfer is the transfer of heat from the high temperature zone to the low temperature zone.First,to strengthen the heat transfer.Such as heat transfer in various heat exchanger.Second,to weaken the heat trans
3、fer.Such as the thermal insulation of equipment to reduce the heat loss.The requirements of heat transfer in chemical productionPrinciples of Chemical Engineering-CDUTThe basic way of heat transferHeat conduction（Conduction）：If there is no relative displacement between the parts of an object,the hea
4、t transfer caused only by the thermal motion of micro particles such as molecules,atoms and free electrons is called heat conduction(also called conduction).The temperature difference between two parts of the system serves as the prerequisite of the heat conduction.Principles of Chemical Engineering
5、-CDUTThe basic way of heat transferHeat conduction occurs in solids,liquids,and gases,butin different ways.In metal solid,heat conduction is caused by themovement of free electrons;In the solid of poor conductorand most of liquid,heat conduction is realized by thevibration of lattice structure,that
6、is,the vibration of atomsand molecules near the equilibrium position;In gases,heat conduction is caused by the irregularmotion of molecules.Principles of Chemical Engineering-CDUTThe basic way of heat transferThe heat transfer caused by the macroscopic motion of particles in the fluid is called conv
7、ective heat transfer.According to the causes of convection,it can be divided into natural convection and forced convection.Heat convectionConvective heat transferIn chemical production,it especially refers to the heat transfer process between fluid and solid wall.Principles of Chemical Engineering-C
8、DUTThe basic way of heat transferIt refers to the phenomenon that an object transmits energy in the form of electromagnetic waves.Heat radiation1.It does not require any medium and can travel through a vacuum.2.It can not only transfer energy,but also the energy form.3.Any object above absolute zero
9、 can emit radiant energy,but only when the temperature of the object is high and the temperature difference between the objects is large can heat radiation become the main heat transfer method.FeaturesPrinciples of Chemical Engineering-CDUTFirst、direct contact heat transfer and hybrid heat exchanger
10、Second、regenerative heat transfer and recuperatorThird、indirect contact heat transfer and recuperative heat exchanger.Heat exchange between cold and thermal fluid and its heat exchangerPrinciples of Chemical Engineering-CDUTThe process of heat transfer between internal cooling and hot fluid of recup
11、erative heat exchanger includes the following three steps:（1）The heat is transferred from the thermal fluid to the pipe wall through heat convection;（2）The heat is transferred from one side of the pipe wall to another side through heat conduction;（3）The heat transferred to the other side is transfer
12、red to the cold fluid by heat convection.Heat exchange between cold and thermal fluid and its heat exchangerPrinciples of Chemical Engineering-CDUTFigure1 telescopic heat exchanger1-inner pipe 2-outer pipeHeat exchange between cold and thermal fluid and its heat exchangerPrinciples of Chemical Engin
13、eering-CDUTHeat exchange between cold and thermal fluid and its heat exchangerFigure 2 single-pass tube and shell heat exchanger1-shell,2-bundle,3、4-nozzle,5-head,6-tubesheet,7-baffle,8-discharge poolPrinciples of Chemical Engineering-CDUTIn chemical production,when the material is heated or cooled
14、in the heat exchanger,it is usually necessary to use another fluid to supply or take away the heat for it.This fluid is called the heat carrier.The heating medium(or heating reagent)plays a heating role and the cooling medium(or coolant)plays a cooling(condensation)role.Heat carrierHeat carrier and
15、its selectionPrinciples of Chemical Engineering-CDUTHeat carrier and its selectionThe selection of heat carriers is usually based on the following four principles:（1）Easy to adjust and control the tempatature.（2）Low saturated vapor pressure,resistant to degradation under heating.（3）Less toxic,non flammable and explosive,low in corrosion（4）Cheap and easy to obtainThe heating medium commonly used in industry includes hot water,saturated steam,mineral oil,diphenyl mixture,molten salt,flue gas,etc;The most commonly used cooling medium include water,air and various refrigerants.